selfhost: unstale the localadd @-prefix doc for the #44 size-keyed tagscr (#44 review)

The header still cited cstage's retired cg_tagscr single cache and
listed the scratch by its pre-#44 name; the grow-fatal mechanism doc
stays (it still guards @retscr et al.) with a pointer to how #44
sidesteps it for the tagged scratch.

selfhost/cmd/w6c/main.combined.ww

@@ -33246,16 +33246,17 @@ fn localadd(c: *cgen, name: str, sz: i32, tnode: *node) i32 = {
 	// would force a later outer `let a: [128]u8` onto the 8B slot,
 	// and `a[127]` would write at +119(BP), past the saved RIP.
 	//
-	// `@`-prefix scratch slots (`@tagscr`, `@retscr`, `@tagbase`,
+	// `@`-prefix scratch slots (`@tagscr<sz>`, `@retscr`, `@tagbase`,
 	// `@sretarg`, `@sretscr`, `@match_spill`, `@vararg_*`) share
 	// one slot per name per fn. Post #15/#26c the slot is sized
 	// at first use and reused by every later caller; a later
 	// caller asking for a larger slot than the first allocation
 	// pinned fatals (rule 7 — surface, don't silently corrupt
 	// the frame: the pinned offset already neighbours other
-	// locals so the slot can't grow in place). Mirrors cstage's
-	// cg_tagscr / cg_retscr / cg_sretscr same-fn caches in
-	// cmd/w6c/cgen.c (#26 / #15).
+	// locals so the slot can't grow in place; #44 sidesteps the
+	// fatal for the tagged scratch by keying its NAME by size).
+	// Mirrors cstage's cg_tagscr_slot table / cg_retscr /
+	// cg_sretscr same-fn caches in cmd/w6c/cgen.c (#26 / #15 / #44).
 	if (name.len > 0) {
 		if (name[0] == '@') {
 			let asz: i32 = sz;

selfhost/cmd/wcc/cgen.ww

@@ -603,16 +603,17 @@ fn localadd(c: *cgen, name: str, sz: i32, tnode: *node) i32 = {
 	// would force a later outer `let a: [128]u8` onto the 8B slot,
 	// and `a[127]` would write at +119(BP), past the saved RIP.
 	//
-	// `@`-prefix scratch slots (`@tagscr`, `@retscr`, `@tagbase`,
+	// `@`-prefix scratch slots (`@tagscr<sz>`, `@retscr`, `@tagbase`,
 	// `@sretarg`, `@sretscr`, `@match_spill`, `@vararg_*`) share
 	// one slot per name per fn. Post #15/#26c the slot is sized
 	// at first use and reused by every later caller; a later
 	// caller asking for a larger slot than the first allocation
 	// pinned fatals (rule 7 — surface, don't silently corrupt
 	// the frame: the pinned offset already neighbours other
-	// locals so the slot can't grow in place). Mirrors cstage's
-	// cg_tagscr / cg_retscr / cg_sretscr same-fn caches in
-	// cmd/w6c/cgen.c (#26 / #15).
+	// locals so the slot can't grow in place; #44 sidesteps the
+	// fatal for the tagged scratch by keying its NAME by size).
+	// Mirrors cstage's cg_tagscr_slot table / cg_retscr /
+	// cg_sretscr same-fn caches in cmd/w6c/cgen.c (#26 / #15 / #44).
 	if (name.len > 0) {
 		if (name[0] == '@') {
 			let asz: i32 = sz;

selfhost/cmd/wwdump/main.combined.ww

@@ -33246,16 +33246,17 @@ fn localadd(c: *cgen, name: str, sz: i32, tnode: *node) i32 = {
 	// would force a later outer `let a: [128]u8` onto the 8B slot,
 	// and `a[127]` would write at +119(BP), past the saved RIP.
 	//
-	// `@`-prefix scratch slots (`@tagscr`, `@retscr`, `@tagbase`,
+	// `@`-prefix scratch slots (`@tagscr<sz>`, `@retscr`, `@tagbase`,
 	// `@sretarg`, `@sretscr`, `@match_spill`, `@vararg_*`) share
 	// one slot per name per fn. Post #15/#26c the slot is sized
 	// at first use and reused by every later caller; a later
 	// caller asking for a larger slot than the first allocation
 	// pinned fatals (rule 7 — surface, don't silently corrupt
 	// the frame: the pinned offset already neighbours other
-	// locals so the slot can't grow in place). Mirrors cstage's
-	// cg_tagscr / cg_retscr / cg_sretscr same-fn caches in
-	// cmd/w6c/cgen.c (#26 / #15).
+	// locals so the slot can't grow in place; #44 sidesteps the
+	// fatal for the tagged scratch by keying its NAME by size).
+	// Mirrors cstage's cg_tagscr_slot table / cg_retscr /
+	// cg_sretscr same-fn caches in cmd/w6c/cgen.c (#26 / #15 / #44).
 	if (name.len > 0) {
 		if (name[0] == '@') {
 			let asz: i32 = sz;